The Gram-negative bacterium, Actinobacillus actinomycetemcomitans, is believed to be the etiologic agent for localized juvenile periodontitis (LJP), a particularly destructive disease in adolescents. The incidence of LJP varies among demographic groups and disproportionately burdens minorities and the poor. A. actinomycetemcomitans has been associated with a variety of other infections, notably brain abscesses, and it is a member of the clinically important HACEK group of bacteria implicated in infective endocarditis. A striking characteristic of fresh clinical isolates of A. actinomycetemcomitans is their ability to form extremely tenacious biofilms, a property thought to be critical for colonization of teeth and other surfaces. Molecular genetic studies in this laboratory have revealed that the genome of A. actinomycetemcomitans maintains a cluster of tad genes required for tight adherence to surfaces. The studies indicate that the tad genes are part of a locus of 14 genes encoding a novel secretion system for the assembly and release of long, bundled FIp fibrils and that the fibrils are required for tight nonspecific adherence to surfaces and bacterial autoaggregation. Remarkably similar tad-like loci were subsequently found in the genome sequences of a wide variety of Gram-negative and Gram-positive Bacteria, including many significant pathogens, and in Archaea. Given the clear requirement of the tad gene cluster for adherence of A. actinomycetemcomitans, the tad loci in other organisms are likely to be important for microbial colonization in a variety of environmental niches. Proposed here are molecular and genetic studies of the tad locus of A. actinomycetemcomitans. The objectives are the following: 1) to understand the mechanisms of expression and regulation of the genes of the tad locus; and 2) to determine the locations, interactions, and molecular functions of the gene products in secretion and fibril assembly. These studies are expected to lead to a basic understanding of a novel secretion system of bacteria and its role in the biogenesis of fibrils required for tight adherence and colonization by A. actinomycetemcomitans. Since of the widespread nature of the tad loci, these studies should also lead to new insights into colonization by other bacterial pathogens and may serve to identify new targets for development of antibiotics.